Find teh entire circumference using Circumference=pi*d. Then, since you are onlky wanting the length of arc XPY and not XY (which is 1/4 of the entire circumfenrce since its at a 90 degree angle), multiple the entire circumference by 3/4

If that's teh case, the answer is 15pi. Since diamter times pi = (10*2)pi. And since you can exclude arc XY (1/4th of the circumference since 90/360 is 1/4) because the question is asking for the arc from point XPY, multiple 20pi by 3/4 to get 15pi.